1. Field of the Invention
The present invention relates to an optical subassembly, in particular, the invention relates to an arrangement of a bi-directional optical subassembly that provides functions of an optical transmitting and an optical receiving for the single optical fiber.
2. Related Prior Art
The bi-directional optical sub-assembly has been well known in the filed of the optical communication system. The BOSA has a function of the optical transmitting for a wavelength of 1.31 μm and a function of the optical receiving for a wavelength of 1.48 μm or 1.55 μm, which is called as the bi-direction optical module (BOSA).
FIG. 1A schematically illustrates one type of the BOSA disclosed in the United States patent application published as US 20060039657A. The BOSA shown in FIG. 1A is implemented with a light transmitting device 110, a light receiving device 120 and a wavelength division multiplexing (hereafter denoted as WDM) unit 130. The light transmitting device 110 installs a semiconductor laser diode (hereafter denoted as LD); while, the light receiving device 120 installs a semiconductor photodiode (hereafter denoted as PD). These devices, 110 and 120, are assembled with the WDM unit 130 with a co-axial shape. On an end of the WDM unit 130 is provided with a sleeve that receives an external optical connector, or, when this BOSA 100 is to be coupled with external equipments via a pig-tail coupling, a pig-tail unit 140.
The WDM unit 130 encloses a WDM filter 132 and a cut filter 134. The light emitted from the light transmitting device 110 couples with the optical fiber within the pig-tail unit 140 after it is concentrated by a lens set on the top of the light transmitting device 110 and passes through the WDM filter 132. While, the light provided from the optical fiber in the pig-tail unit 140 transmits to the light receiving device 120 set in a side of the WDM unit 300 after it is reflected by the WDM filter 132. Although FIG. 1A does not explicitly illustrate another lens set on the top of the light receiving device 120, the received light provided from the pig-tail unit 140 is concentrated on the PD in the light receiving device 120 by this lens.
FIG. 1B is a perspective drawing, which is partially broken to illustrate an inside of the WDM unit 130. As mentioned above, the WDM unit 130 has the co-axial shape with an opening 130a in one end thereof to receive the light transmitting device 110, while, one side thereof attaches the light receiving device 120. One side 130b of the WDM unit 130 opposite to the side where the light transmitting module 110 is set thereon is formed in flat to set the pig-tail unit 140 thereon. The opening 130a provides three bores, 130c to 130e, through which the light emitted from the light transmitting device 110 passes to heads for the WDM filter 132.
The opposite side of the three bores, 130c to 130e, forms a slope 130f to mount the WDM filter 132 thereon. That is, the slope 130f constitutes one of side surfaces not formed in an interior surface constituting the bores, 130c to 130e. FIG. 1A shows this slope 130f viewed from the outside of the WDM unit 130. The slope 130f is necessary to make an angle 45° to an axis connecting the optical axis of the fiber in the pig-tail unit 140 with the axis of the light transmitting device 110 to couple the light from the light transmitting device 110 with the optical fiber and, at the same time, to couple the light from the optical fiber with the light receiving device 120.
When the WDM filter 132 is a type of the multi-layered dielectric film, the transmittance and the reflectance of the WDM filter 132 depends on the incident angle of the light. In the BOSA 100 shown in FIGS. 1A and 1B, the WDM filter 132 must transmit the light with the wavelength of 1.31 μm in roughly 100%, while, it must reflect the light with wavelengths of 1.48 to 1.55 μm also in roughly 100%. Accordingly, the angle between the optical axis of the WDM filter 132 and that of the light (the incoming light or the outgoing light) must be severely adjusted.
The BOSA 100 shown in FIGS. 1A and 1B provides the slope 130f, where the WDM filter 132 is to be mounted, processed by, for instance, the milling machine. Specifically, setting the WDM unit 130 in the machine such that the axis thereof is inclined by 45° with respect to the axis of the milling drill, the slope 130f is processed in flat. Thus, the conventional BOSA 100 is necessary to process the structure to mount the WDM filter 132 from the outside by using special tools.